Conventionally, there have been disclosed techniques for fabricating a boron phosphide-based semiconductor device such as a light-emitting diode (abbreviated as LED) from boron phosphide (chemical formula: BP), which is a type of Group III-V compound semiconductor, and a mixed crystal thereof (see, for example, U.S. Pat. No. 6,069,021). For example, a p-conduction-type boron monophosphide (chemical formula: BP) layer is employed to serve as a barrier layer constituting a light-emitting portion having a pn-doublehetero (DH) junction structure (see, for example, Japanese Laid-Open Patent Application (kokai) No. 2-288388). A boron phosphide-based semiconductor light-emitting diode is composed of, for example, a p-type cladding layer formed of a boron phosphide layer, and a p-type Ohmic electrode provided on the surface of the p-type cladding layer. In one conventional case, a p-type Ohmic electrode provided on a p-type boron phosphide layer is formed from aluminum (Al) (see, for example, K. Shohno et al., J. Crystal Growth, Vol. 24/25, 1974 (the Netherlands), p. 193).
Boron phosphide is known to provide either an n-conduction-type or a p-conduction-type low-resistance semiconductor layer, even when no impurity is intentionally added thereto (see, for example, K. Shohno et al., J. Crystal Growth, Vol. 24/25, 1974 (The Netherlands), p. 193). Thus, an Ohmic electrode can be formed on a conductive boron phosphide layer such as a cladding layer or a contact layer. In a conventional compound semiconductor light-emitting device having a magnesium (symbol of element: Mg)-doped p-type boron phosphide layer serving as a contact layer, an Ohmic contact electrode is formed from gold (symbol of element: Au)-zinc (symbol of element: Zn) (as disclosed in, for example, Japanese Laid-Open Patent Application (kokai) No. 2-288388).
However, when the aforementioned metallic species are employed, formation of an Ohmic electrode exhibiting excellent Ohmic contact properties with respect to p-type boron phosphide has not been successfully achieved. Therefore, input resistance upon passage of an electric current supplied for driving a light-emitting device (i.e., device operation current) disadvantageously increases, resulting in an LED exhibiting high forward voltage (Vf), which is problematic. Such high input resistance is also problematic for producing a laser diode (LD) having a low threshold voltage (Vth).
An object of the present invention is to provide a p-type Ohmic electrode structure, to provide excellent Ohmic contact properties, of a p-type Ohmic electrode on the surface of a p-type boron phosphide-based semiconductor layer containing boron (B) and phosphorus (P) as constitutional elements. The term “p-type Ohmic electrode” refers to a positive electrode which is provided on a p-type semiconductor layer. Another object of the present invention is to provide a compound semiconductor light-emitting device having a p-type Ohmic electrode having the electrode structure according to the present invention.